June 24 is the Feast of the Birth of Saint John the Baptist, the cousin of Christ who leaped in his mother Elizabeth's womb at the Visitation, when the Virgin Mary came to share with her the good news she had been given at the Annunciation. Christians, of course, normally commemorate the day that a saint died, not the day that he was born, because his death represents his rebirth into eternal life. There are only three exceptions to this rule: Christ (which is why we celebrate Christmas); the Virgin Mary (whose nativity we celebrate on September 8, nine months after the Feast of the Immaculate Conception); and St. John the Baptist.
It would be natural to think that the reason exceptions are made for Christ and the Blessed Virgin is because of their central importance to the mystery of our salvation. But in fact, the reason why we celebrate their births is the same reason why we celebrate the nativity of John the Baptist. Traditionally, Catholics have seen John the Baptist's leap in his mother's womb as a type of Baptism, and thus believe that he is one of only three people born without Original Sin--Christ Himself and the Blessed Virgin being the other two. The difference, of course, is that both Christ and the Virgin Mary were conceived without sin, while John the Baptist was conceived in sin but cleansed before his birth. (See What Is the Immaculate Conception? for more on God's preservation of Mary from Original Sin.)
John the Baptist dedicated his life to calling the Jews to repentance. He is called the Forerunner of Christ, because he "prepared the way of the Lord" through his "voice crying out in the wilderness." In traditional iconography, he is depicted with wings like an angel, because angels are messengers between God and man.